1. Field of the Invention
The present invention relates to a control system for a compression ignition internal combustion engine.
2. Description of Related Art
For example, some compression ignition internal combustion engines, such as diesel engines, have direct fuel injection valves that inject fuel into corresponding cylinders. The fuel, which is injected from each fuel injection valve into the corresponding cylinder, is combusted together with intake air. In the above type of internal combustion engine, there are implemented multiple combustion modes, which have different fuel injection timings that are set in view of the engine output characteristics and exhaust gas characteristics. One of the fuel combustion modes is selected based on the rotational speed and the load of the engine. The fuel injection timing, the fuel injection quantity, the intake air quantity and the recirculation quantity of the exhaust gas through an exhaust gas recirculation system are controlled for each combustion mode. For example, in the conventional combustion, a premixed combustion period and a diffusion combustion period exist. In the premixed combustion period, the fuel and the air are mixed together during an ignition delay period, and then the premixed combustion of this air-fuel mixture takes place. In the diffusion combustion period, the injected fuel is immediately combusted right after the fuel injection. In contrast to the conventional combustion, in a lately developed premixed combustion, the control operation is performed such that the oxygen concentration is set to be a relatively low value due to supply of a large quantity of the EGR gas, and the ignition timing does not occur in the fuel injection period. In the following description, this widely known premixed combustion will be referred to as complete premixed combustion. Besides the complete premixed combustion, a semi-premixed combustion may be implemented in an intermediate range in an engine operational range between the complete premixed combustion and the conventional combustion, as shown in FIG. 2, to achieve smooth change between the complete premixed combustion and the conventional combustion. In the semi-premixed combustion, each of the oxygen concentration, the fuel injection timing and the fuel injection quantity are set to be a corresponding intermediate value between the corresponding value in the complete premixed combustion and the corresponding value in the conventional combustion.
During the operation of the engine, a change in the fuel injection system and a change in the air system show different responses relative to a change in its corresponding target. Specifically, this is due to the following differences between the fuel injection system and the air system. That is, in the fuel injection system, the fuel injection timing and the fuel injection quantity may be instantaneously adjusted by changing the fuel injection mode. In contrast, in the air system, an actuation delay of an actuator(s) and a delay in conduction of a flow may occur. Because of this, the balance between the fuel injection system and the air system is deteriorated at the time of changing the combustion mode, so that the characteristics of the exhaust gas may be deteriorated, and the shaft torque may be changed. This may cause a deterioration of the drivability of the vehicle.
Japanese Unexamined Patent Publication Number 2005-48724 (corresponding to US 2005/0022517 A1) discloses a control method that addresses the above disadvantage. According to this control method, when a target value of an excess air ratio significantly changes, a ratio between an amount of change in the target value of the excess air ratio and a difference between the target value of the excess air ratio and an actual value of the excess air ratio is obtained. Based on this ratio, the fuel injection timing is corrected. FIG. 6A shows a way of correcting the fuel injection timing by applying the above control method in a case where the target value of the excess air ratio is changed at the timing ta. In FIG. 6A, the amount of change in the target value of the excess air ratio will be denoted by A1, and the amount of change in the target value of the fuel injection timing is denoted by B1. Furthermore, a difference between the target value of the excess air ratio and the actual value of the excess air ratio is denoted by A2, and a difference between the target value of the fuel injection timing and the actual value of the fuel injecting timing is denoted by B2. The fuel injection timing is corrected in a manner that satisfies a relationship of A1:A2=B1:B2.
The inventors of the present invention found a relationship between the excess air ratio (corresponding to oxygen information) and the fuel injection timing (corresponding to ignition timing) for achieving the generally equal constant shaft torque of the engine. FIG. 6B shows the relationship between the excess air ratio and the fuel injection timing for achieving the generally equal constant shaft torque where the control method disclosed in Japanese Unexamined Patent Publication Number 2005-48724 is applied.
In FIG. 6B, a characteristic curve L3 indicates the relationship between the excess air ratio and the fuel injection timing for achieving the generally equal constant shaft torque before changing of the combustion mode from the conventional combustion to the complete premixed combustion, and a characteristic curve L4 indicates the relationship between the excess air ratio and the fuel injection timing for achieving the generally equal constant shaft torque after the changing of the combustion mode from the conventional combustion to the complete premixed combustion. As discussed above, the EGR quantity in the complete premixed combustion is larger than the EGR quantity in the conventional combustion, so that the characteristic curves L3, L4 differ from one another. The target value of the excess air ratio and the target value of the injection timing are located at a point D of the characteristic curve L3 before the timing ta, at which the combustion mode is changed from the conventional combustion to the complete premixed combustion. However, after the timing ta, the target value of the excess air ratio and the target value of the injection timing are located at a point E of the characteristic curve L4. As discussed above, the change in the air system is delayed from the change in the fuel injection system, so that the fuel injection timing is corrected to satisfy the relationship of the A1:A2=B1:B2. Thus, the excess air ratio and the fuel injection timing at the arbitrary timing tb are located at a point F, which is deviated from the characteristic curve L4 in the combustion mode after the changing from the conventional combustion to the complete premixed combustion. Therefore, the combustion is changed to have the different shaft torque that is different from the shaft torque before the changing of the combustion mode. As a result, the drivability of the vehicle is deteriorated.